1. Field of the Invention
The present invention relates to a switchback mechanism to reverse a paper sheet conveying direction, a switchback apparatus provided with at least one switchback mechanism and a bypass conveying path to bypass the switchback mechanism, and a switchback method.
2. Description of the Related Art
There is a conventional switchback apparatus, which accepts a paper sheet conveyed in a first direction in a nip of a switchback roller rotating in a first direction, feeds the sheet in the first direction, stops once while holding the sheet in the nip, and reverses the sheet conveying direction by rotating the switchback roller in a second direction opposite to the first direction (refer to JP-A 2004-175507 (KOKAI).
As illustrated in FIG. 14, this conventional switchback apparatus has an area 200 not tightly holding a sheet M in the downstream of a nip N along a first direction (left to right in the drawing). Usually, the length of this area 200 is designed shorter than a shortest sheet among sheets M to be handled (hereinafter, such a sheet is called a shortest sheet Mmin). Namely, the length of the area 200 is designed so that the sheets M of all lengths fed to a switchback apparatus do not come out of the nip and drop into the apparatus.
However, in an actual operation, a sheet M may be creased or torn on the way of conveying, and become shorter than an assumed shortest sheet Mmin. Even if such a sheet M becoming short undesirably is conveyed to a switchback apparatus, the sheet can be rejected without being input to a switchback apparatus, when the existence of that sheet M can be detected before it is fed to the switchback apparatus. But, if a short sheet M overlaps with a normal sheet M and fed to a switchback apparatus together with the normal sheet M, the short sheet M may remain in the area 200 not having a feeding force in a switchback apparatus, after the normal sheet M is output from a switchback apparatus.
If a normal-length sheet M is fed to a switchback apparatus while a short sheet M undesirably remains in the area 200 (hereinafter, such a sheet remaining in an area not having a tight-holding force is called a remaining sheet Mr), the sheet M abuts the sheet Mr, and the sheets may be creased and are very likely to cause a paper jam. In this case, the remaining sheet Mr and normal sheet M fed after may tear. When a paper jam occurs in a switchback apparatus, it is necessary to stop the apparatus and clear the jam. This reduces the throughput of the apparatus.
Moreover, if such a torn sheet M is fed out from a switchback apparatus, the sheet may interfere with other normally-handled sheets M and cause secondary trouble. For example, a remaining sheet Mr output from a switchback unit at an uncontrollable timing overlaps with a normally-handled sheet M at a joint point after passing along another path, and causes another paper jam. Namely, if a remaining sheet Mr remaining in a switchback apparatus is output from the switchback apparatus at an unexpected timing, it causes another paper jam, and requires stopping the apparatus to clear the jam, decreasing the throughput of the apparatus.